1. Field of the Invention
The present invention relates to a phosphor having good emission characteristics for use in various display devices such as electroluminescent display (EL) panels or the like, and an apparatus for and a method of manufacturing such a phosphor with high reproducibility.
2. Description of the Related Art
Phosphors for use in EL panels, for example, are usually made of zinc sulfide (ZnS) with an activator of copper (Cu), a coactivator of bromine (Br), and a coactivator of chlorine (Cl) added thereto. Since the service life of phosphors and the brightness thereof upon emission depend greatly upon the concentrations of the activator and coactivators added and the particle size of the phosphors, it is important that the amounts of the activator and coactivators added, i.e., the concentrations of impurities added, be controlled appropriately.
To manufacture such a phosphor for use in an electroluminescent panel, hereinafter referred to as an EL phosphor, as shown in FIG. 1 of the accompanying drawings, it has heretofore been customary to add, to a material 30 of zinc sulfide, 0.1.about.1.0 mol % of a copper salt such as copper acetate or copper sulfate, and 0.1.about.20 mol % of ammonium bromide or ammonium chloride, mix the materials well, store the mixture in an ampul 32 of quartz or the like closed by a cap 31, place the ampul 32 in a reactor 33 of an electric furnace 35, and energize a heater 34 of the electric furnace 35 with the reactor 33 being filled with a hydrogen sulfide (H.sub.2 S) or nitrogen (N.sub.2) gas atmosphere for thereby firing the mixture at a temperature ranging from 700.degree. to 1200.degree. C. until the mixture is turned into particles of suitable size.
Because excessive copper or the like is attached to the surface of the EL phosphor thus manufactured, the EL phosphor is then washed with a solution of sodium cyanide or the like and dried into an EL phosphor product.
In the conventional manufacturing apparatus, ammonium bromide or ammonium chloride is added as a coactivator for copper and also as a promoter for increasing the particle size of the EL phosphor. Since ammonium bromide and ammonium chloride have low subliming points of 542.degree. C. and 337.8.degree. C., respectively, they would sublime almost entirely at the firing temperature ranging from 700.degree. to 1200.degree. C.
To avoid such unwanted sublimation, zinc sulfide with an impurity such as of ammonium bromide or ammonium chloride added is placed in the ampul 32, and the ampul 32 is closed by the cap 31 to prevent the added ammonium bromide or ammonium chloride from subliming while the particle size of the EL phosphor is increasing, i.e., the crystal of the EL phosphor is growing.
The conventional manufacturing apparatus which employs the ampul 32 closed by the cap 31 has had a problem because the manufactured EL phosphor suffers particle size irregularities and impurity concentration variations depending on the shape and volume of the ampul 32 and the gap between the ampul 32 and the cap 31. Therefore, the conventional manufacturing apparatus fails to manufacture EL phosphors with high reproducibility, and EL phosphors manufactured by the conventional manufacturing apparatus have widely different emission characteristics.